Electro-wetting on dielectric (EWOD) is a well known technique for manipulating droplets of fluid by application of an electric field. Active Matrix EWOD (AM-EWOD) refers to implementation of EWOD in an active matrix array incorporating transistors, for example by using thin film transistors (TFTs). It is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007) 3:245-281).
FIG. 1 shows a part of a conventional EWOD device in cross section. The device includes a lower substrate 72, the uppermost layer of which is formed from a conductive material which is patterned so that a plurality of array element electrodes 38 (e.g., 38A and 38B in FIG. 1) are realized. The electrode of a given array element may be termed the array element electrode 38. The liquid droplet 4, comprising a polar material (which is commonly also aqueous and/or ionic), is constrained in a plane between the lower substrate 72 and a top substrate 36. A suitable gap between the two substrates may be realized by means of a spacer 32, and a non-polar fluid 34 (e.g. oil) may be used to occupy the volume not occupied by the liquid droplet 4. An insulator layer 20 disposed upon the lower substrate 72 separates the conductive element electrodes 38A, 38B from a first hydrophobic coating 16 upon which the liquid droplet 4 sits with a contact angle 6 represented by θ. The hydrophobic coating is formed from a hydrophobic material (commonly, but not necessarily, a fluoropolymer).
On the top substrate 36 is a second hydrophobic coating 26 with which the liquid droplet 4 may come into contact. Interposed between the top substrate 36 and the second hydrophobic coating 26 is a reference electrode 28.
The contact angle θ 6 is defined as shown in FIG. 1, and is determined by the balancing of the surface tension components between the solid-liquid (γSL), liquid-gas (γLG) and non-polar fluid (γSG) interfaces, and in the case where no voltages are applied satisfies Young's law, the equation being given by:
                              cos          ⁢                                          ⁢          θ                =                                            γ              SG                        -                          γ              SL                                            γ            LG                                              (                  equation          ⁢                                          ⁢          1                )            
In certain cases, the relative surface tensions of the materials involved (i.e the values of γSL, γLG and γSG) may be such that the right hand side of equation (1) is less than −1. This may commonly occur in the case in which the non-polar fluid 34 is oil. Under these conditions, the liquid droplet 4 may lose contact with the hydrophobic coatings 16 and 26, and a thin layer of the non-polar fluid 34 (oil) may be formed between the liquid droplet 4 and the hydrophobic coatings 16 and 26.
In operation, voltages termed the EW drive voltages, (e.g. VT, V0 and V00 in FIG. 1) may be externally applied to different electrodes (e.g. reference electrode 28, array element electrodes 38, 38A and 38B, respectively). The resulting electrical forces that are set up effectively control the hydrophobicity of the hydrophobic coating 16. By arranging for different EW drive voltages (e.g. V0 and V00) to be applied to different element electrodes (e.g. 38A and 38B), the liquid droplet 4 may be moved in the lateral plane between the two substrates 72 and 36.
In the following description, it will be assumed that an element of an EWOD device, such as the device of FIG. 1, may receive “digital” data so that the element is required to be put in either an “actuated” state in which the voltage applied across the element is sufficient for a liquid droplet in the element (if one is present in the element) to experience a significant electro-wetting force, or a “non-actuated” state in which the voltage applied across the element is not sufficient for a liquid droplet in the element (if one is present in the element) to experience a significant electro-wetting force. An element of an EWOD device may be put into the actuated state by applying a voltage difference across the EWOD element having a magnitude that is equal to, or greater than, a threshold voltage VEW, whereas if the voltage difference across the EWOD element has a magnitude that is less than the threshold voltage VEW the element is in its non-actuated state. The threshold voltage VEW is often referred to as an “actuation voltage”, and this term is used below. In practice, the threshold voltage may typically be determined as the minimum voltage required to effect droplet operations, for example the moving or splitting of droplets. In practice, the non-actuated state may typically be zero volts. Typically EWOD systems may be considered to be digital, in that the EWOD elements are programmed either to an actuated or non-actuated state. It should however be understood that an EWOD device may also be operated by supplying analogue data, such that EWOD elements may be partially actuated.
U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) discloses a passive matrix EWOD device for moving droplets through an array.
U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28, 2005) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions.
U.S. Pat. No. 6,565,727 further discloses methods for other droplet operations including the splitting and merging of droplets, and the mixing together of droplets of different materials.
U.S. Pat. No. 7,163,612 (Sterling et al., issued Jan. 16, 2007) describes how TFT based thin film electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements similar to those employed in Active Matrix (AM) display technologies.
The approach of U.S. Pat. No. 7,163,612 may be termed “Active Matrix Electro-wetting on Dielectric” (AM-EWOD). There are several advantages in using TFT based thin film electronics to control an EWOD array, namely:                Electronic driver circuits can be integrated onto the lower substrate 72.        TFT-based thin film electronics are well suited to the AM-EWOD application. They are cheap to produce so that relatively large substrate areas can be produced at relatively low cost.        TFTs fabricated in standard processes can be designed to operate at much higher voltages than transistors fabricated in standard CMOS processes. This is significant since many EWOD technologies require EWOD actuation voltages in excess of 20V to be applied.        
A disadvantage of U.S. Pat. No. 7,163,612 is that it does not disclose any circuit embodiments for realizing the TFT backplane of the AM-EWOD.
EP2404675 (Hadwen et al., published Jan. 11, 2012) describes array element circuits for an AM-EWOD device. Various methods are known for programming and applying an EWOD actuation voltage to the EWOD element electrode. The programming function described includes a memory element of standard means, for example, based on Dynamic RAM (DRAM) or Static RAM (SRAM) and input lines for programming the array element.
Whilst EWOD (and AM-EWOD) devices can be operated with either DC or AC actuation voltages, in practice there are many reasons for preferring an AC method of driving, as reviewed in the previously cited reference R. B. Fair, Microfluid Nanofluid (2007) 3:245-281). It may be noted that droplets can be actuated and manipulated for a wide range of AC driving frequencies ranging typically from a few hertz to several kHz.
U.S. Pat. No. 8,653,832 (Hadwen et al., issued Feb. 18, 2014) describes how an impedance (capacitance) sensing function can be incorporated into the array element. The impedance sensor may be used for determining the presence and size of liquid droplets present at each electrode in the array.
UK Application GB1500261.1, which is herein incorporated by reference, describes a two transistor (2T) array element circuit and a method of driving for implementing an AC driving method of driving. The 2T array element actuation circuit disclosed is shown in FIG. 2 of the current application. This UK application further includes an embodiment showing how the impedance (capacitance) sensing function of U.S. Pat. No. 8,653,832 can be combined with the 2T array element actuation circuit. The array element circuit including the sensor function is shown in FIG. 3 and contains a total of 5 transistors, 3 capacitors and 9 addressing lines. Addressing lines DATA and ENABLE control access to a Dynamic RAM memory circuit comprising the transistor to which they are connected and a capacitor. The voltage programmed to this capacitor in turn controls whether or not the input signal ACTUATE is connected through to an array element electrode. The input signal SEN may further be used to isolate the element electrode from the ACTUATE signal when the sensor is being operated. The sensor function is controlled by two voltage signals applied to terminals RWS and RST. The voltage signal applied to RST resets the voltage at the gate of a sense transistor (connected between VDD and COL) to a reset potential VRST. The voltage signal applied to RWS perturbs the voltage at the element electrode by an amount dependent on the ratio of the fixed capacitors in the circuit present at the element electrode and the capacitance presented by the presence or absence by a liquid droplet at the element electrode. A voltage signal is thus coupled to the gate of the sensing transistor which is converted to an output current through COL. The impedance presented at the element electrode may thus be measured.